Manufacture and construction of



Feb. 21, 1956 J, T. CRANDALL 2,735,258

MANUFACTURE AND CONSTRUCTION OF STRINGS Filed Aug. 9, 1951 5 lKKKfiKfiKKQK- 1(((((((((((((\((((((((( L BY yuizw 7 233;; w/w 1 4 ATTORNEY United States Patent MANUFACTURE AND CUNSTRUCTION 0F STRINGS dulian T. Qrandall, Ashaway, R. l.

Application August 9, 1951, Serial No. 241,055

Claims. ((157-140) The present invention relates to the manufacture of strings for stringing tennis, badminton and squash racquets and for musical instruments, and has particular reference to a novel manufacture and a novel construction therefor.

The principal object of the invention is to provide an improved string for tennis racquets, musical instruments, and the like.

Another object of the invention is to provide an improved string comprising a filament core and an integrally joined sheath.

A further object of the invention is to provide an improved string having great strength, elasticity, and resistance to fatigue and distortion.

With the above and other objects and advantageous features in view, the invention consists of a novel method and a novel article more fully discolsed in the detailed description following, in conjunction with the accompanying drawings, and more specifically defined in the claims appended thereto.

In the drawings,

Fig. l is a perspective view of a tennis racquet strung with the improved string;

Fig. 2 is an enlarged portion of the string, parts being broken away to show the elements thereof; and

Pig. 3 is a perspective enlarged view showing the appearance of the string during the ditferent stages of manufacture.

It has been found desirable to provide an improved string for tennis racquets, and the like, which has great strength, is very resilient, has great resistance to tension strains and to flexing, and has a smooth water repellent outer surface. To this end, I have devised a composite construction which utilizes a center flexible core of high resiliency, preferably made of linear synthetic plastic filaments, and a tough composite armor sheath, preferably including a textile winding and a synthetic plastic filament winding, the sheath being integrally locked to the core to provide a smooth, hard, abrasion resisting string and having an outer smooth coating.

1 have found that a suitable plastic for such strings is the amide-polymer, vinylite and polyester types, and that nylon is particularly desirable because it orients under tension when drawn down to the desired diameter to increase the strength of the string.

Referring to the drawings, Fig. 1 illustrates a racquet it) which has been strung with the improved string 11. The string ll. as shown in Fig. 2, has a linear filament core 1" which is flexible and of high resiliency and an armor sheath 13, which is tough and wiry, the core and the sheath being integrally locked, preferably with synthetic plastic 14. The resulting string has an integrated linear core and a wound sheath with a smooth outer coating, is hard and tough, and does not abrade when pulled through the string holes of the racquet.

The improved string is manufactured by first forming a core of linear strands as indicated by the reference numeral 15 in Fig. 3. This core is passed through a 2,735,258 Ratented .Feb. 21:, 1956 tank containing a solution of a special coating material, which preferably has a nylon base, it being preferred to apply several thin layers of a coating rather than a single heavier layer. The coated core may be lightly brushed to obtain a smooth finish, as indicated by the reference numeral 16, and then air dried, and textile thread is wound spirally over the coated core as indicated by the reference numeral 17. The covered core is again coated by passing through the integrating solution, preferably to obtain several thin layers, and may be again lightly brushed to provide a smooth coated covered core as indicated by the reference numeral .18. The coated textile wound core is now air dried; .a second spiral Winding 19 of a plastic thread, preferably nylon monofilament, is wound over the smoothed coating -18, and is coated in the integrating solution, and lightly brushed to smooth the surface as indicated at 20. The assembly is now stretched under tension and under predetermined heat conditions, to reduce the diameter and form a complay, and the preferred winding is of a close nature with the filament coils touching each other. When the string is manufactured for musical instruments, the number and extent of the coatings are regulated to provide a desired tone.

Although I have found that synthetic resin solutions may be used for integrating the string, a satisfactory coating solution includes nylon, dissolved with isopropanol and tetrahydrofurfuryl alcohol in a water base, the proportion of nylon being variable between the limits of 10 to 25% by weight, of isopropanol between the limits of 50 to 70% by weight, and of tetrahydrofurfuryl alcohol between the limits of 5 to 20% by weight, the proportions being chosen and sufficient water being added to provide a coating solution of suitable consistency.

Two specific formulae that have been found suitable for the desired coatings are as follows, based on a sixty pound batch:

Formula N 0. 1

ing solution, and the proportions of the ingredients are varied to provide a desired stiffness in the final product. For strings requiring more flexibility, it is desirable to add a predetermined amount of plasticizer. It has been found that a string which has a diameter of .056 inch before integration will break under a dead load of one hundred pounds; after integration, with a diameter reduced to .050 inch, the break load has risen to one hundred ten pounds.

The integration drawing for a string of the size described is preferably under a strain load of fifty pounds, using a drawing speed of about eight hundred yards per hour and a temperature of substantially 360 F. The resulting string has the stillness suitable for use in tennis racquets and the like. Change in drawing speed, pull and temperature will produce a variation in the stiffness; the range of speed is from two hundred to one thousand yards per hour, and the range of temperature is from 250 to 400 F.

A string made as described has high tensile strength, very high resiliency when strung, and great resistance to fatigue and distortion, whereby the racquet strings hold their shape, give long wear, are water repellent, and do not elongate or stretch with continued use. When the string is made of nylon, the linear core provides high resilience, and molecular orientation is produced by the stretching and is fixed by the quick cooling.

For certain string constructions the textile winding may be omitted to provide an all plastic string. The preferred material for the core is linear multi-filament nylon, which has high elasticity and resilience, the individual filaments being as small as .002 inch in diameter. The preferred material for the outer spiral layer is monofilament nylon of larger cross-section, such as .008 inch in diameter, which produces a hard, tough wearing surface, the spiral winding having substantially no efiect on the resiliency of the multi-filament core although integrated with the core.

Other suitable plastic monofilament material of the polyamide, Vinylite and polyester type may be used for composite strings for different uses, with textile and silk cores of sheaths, and in combination with plastic materials, suitable binders being utilized to integrate the flexible material core with a cover or sheath of tough material.

Although I have described a specific string construction designed for racquets and musical instruments, it is obvious that the string may be used for other purposes, such as for example for fish-hook leaders, and fishing lines,

and that changes in the size and shape of the parts and in the material used may be made Without departing from the spirit and the scope=of the invention as defined in the appended claims.

I claim:

1. In the manufacture of strings, the steps of forming a core of linear multi-filarnent strands of thermoplastic material, coating the core with a solution of the same thermoplastic material, drying the coated core, winding a monofilament of the same thermoplastic material on the dried coated core to form a spiral layer thereon, coating the spiral layer with a solution of the same thermoplastic material, and drying the spiral layer coating, whereby a composite string results.

2. In the process of claim 1, said thermoplastic material being nylon.

3. In the process of claim 2, subjecting the composite string to stretching under tension and heat to integrate the core, the spiral layer, and the coatings.

4. An integrated string having a core of linear multifilament strands of thermoplastic material, a coating of the same thermoplastic material over the core, a spiral layer of the same thermoplastic material monofilament on said coating, and a second coating of the same thermoplastic material over the spiral layer, the core and layer being locked together by the coatings.

5. In the article of claim 4, the thermoplastic material being nylon.

References Cited in the file of this patent UNITED STATES PATENTS 1,970,376 Hamburger Aug. 14, 1934 2,091,999 Madge et a1 Sept. 7, 1937 2,157,117 Miles, Jr. May 9, 1939 2,311,704 Simison Feb. 23, 1943 2,335,644 Camp Nov. 30, 1943 2,401,291 Smith May 28, 1946 2,521,055 Foster Sept. 5, 1950 2,649,833 Crandall Aug. 25, 1953 

1. IN THE MANUFACTURE OF STRINGS, THE STEPS OF FORMING A CORE OF LINEAR MULTI-FILAMENT STRANDS OF THERMOPLASTIC MATERIAL, COATING THE CORE WITH A SOLUTION OF THE SAME THERMOPLASTIC MATERIAL, DRYING THE COATED CORE, WINDING A MONOFILAMENT OF THE SAME THERMOPLASTIC MATERIAL ON THE DRIED COATED CORE TO FORM A SPIRAL LAYER THEREON, COATING THE SPIRAL LAYER WITH A SOLUTION OF THE SAME THERMOPLASTIC MATERIAL, AND DRYING THE SPIRAL LAYER COATING, WHEREBY A COMPOSITE STRING RESULTS. 